Chapter 1 Contact Angle and Wetting Properties Yuehua Yuan and T. Randall Lee Abstract This chapter highlights a variety of techniques that are commonly used to measure contact angles, including the conventional telescope-goniometer method, the Wilhelmy balance method, and the more recently developed drop-shape analysis methods. The various applications and limitations of these techniques are described. Notably, studies of ultrasmall droplets on solid surfaces allow wetting theories to be tested down to the nanometer scale, bringing new insight to contact angle phenom- ena and wetting behavior. 1.1 Introduction The topic of wetting has received tremendous interest from both fundamental and applied points of view. It plays an important role in many industrial processes, such as oil recovery, lubrication, liquid coating, printing, and spray quenching [1–6]. In recent years, there has been an increasing interest in the study of superhydrophobic surfaces, due to their potential applications in, for example, self-cleaning, nanoflu- idics, and electrowetting [7–12]. Wettability studies usually involve the measure- ment of contact angles as the primary data, which indicates the degree of wetting when a solid and liquid interact. Small contact angles ( 90°) correspond to high wettability, while large contact angles ( 90°) correspond to low wettability. This chapter will begin with an introduction of the fundamental science behind wetting and contact angle phenomena, followed by a comprehensive description of the various techniques used to measure contact angles, as well as their appli- cations and limitations in terms of the geometric forms of solid samples. Most of the techniques can be classified into two main groups: the direct optical method and the indirect force method. Calculations based on measured contact angle val- ues yield an important parameter—the solid surface tension, which quantifies the Y. Yuan · T.R. Lee (B) Department of Chemistry, University of Houston, 4800 Calhoun Road, Houston, TX 77204-5003, USA e-mail: [email protected] Y. Yuan e-mail: [email protected] G. Bracco, B. Holst (eds.), Surface Science Techniques, 3 Springer Series in Surface Sciences 51, DOI 10.1007/978-3-642-34243-1_1, © Springer-Verlag Berlin Heidelberg 2013 4 Y. Yuan and T.R. Lee Fig. 1.1 Illustration of contact angles formed by sessile liquid drops on a smooth homogeneous solid surface wetting characteristics of a solid material. The criteria of calculating solid surface tension based on experimental contact angle values will be discussed. Finally, the most up-to-date contact angle measurement techniques will be presented and dis- cussed. 1.2 Theoretical Background 1.2.1 Surface Tension and Contact Angle—Young’s Equation Consider a liquid drop resting on a flat, horizontal solid surface (Fig. 1.1). The con- tact angle is defined as the angle formed by the intersection of the liquid-solid in- terface and the liquid-vapor interface (geometrically acquired by applying a tangent line from the contact point along the liquid-vapor interface in the droplet profile). The interface where solid, liquid, and vapor co-exist is referred to as the “three- phase contact line”. Figure 1.1 shows that a small contact angle is observed when the liquid spreads on the surface, while a large contact angle is observed when the liquid beads on the surface. More specifically, a contact angle less than 90° indi- cates that wetting of the surface is favorable, and the fluid will spread over a large area on the surface; while contact angles greater than 90° generally means that wet- ting of the surface is unfavorable so the fluid will minimize its contact with the surface and form a compact liquid droplet. For example, complete wetting occurs when the contact angle is 0°, as the droplet turns into a flat puddle. For superhy- drophobic surfaces, water contact angles are usually greater than 150°, showing almost no contact between the liquid drop and the surface, which can rationalize the “lotus effect” [13]. Furthermore, contact angles are not limited to the liquid- vapor interface on a solid; they are also applicable to the liquid-liquid interface on a solid. Ideally, the shape of a liquid droplet is determined by the surface tension of the liquid. In a pure liquid, each molecule in the bulk is pulled equally in every direction by neighboring liquid molecules, resulting in a net force of zero. How- ever, the molecules exposed at the surface do not have neighboring molecules in all directions to provide a balanced net force. Instead, they are pulled inward by the neighboring molecules (Fig. 1.2), creating an internal pressure. As a result, the liquid voluntarily contracts its surface area to maintain the lowest surface free 1 Contact Angle and Wetting Properties 5 Fig. 1.2 Surface tension is caused by the unbalanced forces of liquid molecules at the surface energy. From everyday life, we know that small droplets and bubbles are spheri- cal, which gives the minimum surface area for a fixed volume. This intermolecu- lar force to contract the surface is called the surface tension, and it is responsible for the shape of liquid droplets. In practice, external forces such as gravity deform the droplet; consequently, the contact angle is determined by a combination of sur- face tension and external forces (usually gravity). Theoretically, the contact angle is expected to be characteristic for a given solid-liquid system in a specific environ- ment [14]. As first described by Thomas Young [15] in 1805, the contact angle of a liquid drop on an ideal solid surface is defined by the mechanical equilibrium of the drop under the action of three interfacial tensions (Fig. 1.1): γlv cos θY = γsv − γsl (1.1) where γlv, γsv, and γsl represent the liquid-vapor, solid-vapor, and solid-liquid in- terfacial tensions, respectively, and θY is the contact angle. (1.1) is usually referred to as Young’s equation, and θY is Young’s contact angle. 1.2.2 Contact Angle Hysteresis From Young’s equation applied to a specific liquid-solid system, three thermody- namic parameters γlv, γsv, and γsl determine a single and unique contact angle θY . In practice, however, there exist many metastable states of a droplet on a solid, and the observed contact angles are usually not equal to θY . The phenomenon of wetting is more than just a static state. The liquid moves to expose its fresh surface and to wet the fresh surface of the solid in turn. The measurement of a single static con- tact angle to characterize wetting behavior is no longer adequate. If the three-phase contact line is in actual motion, the contact angle produced is called a “dynamic” contact angle. In particular, the contact angles formed by expanding and contracting the liquid are referred to as the advancing contact angle θa and the receding contact angle θr , respectively (Fig. 1.3). These angles fall within a range, with the advanc- ing angles approaching a maximum value, and the receding angles approaching a minimum value. Dynamic contact angles can be measured at various rates of speed. At a low speed, it should be close or equal to a properly measured static contact angle. The difference between the advancing angle and the receding angle is called the hysteresis (H ): H = θa − θr (1.2) 6 Y. Yuan and T.R. Lee Fig. 1.3 Illustration of advancing and receding contact angles The significance of contact angle hysteresis has been extensively investigated [16–20], and the general conclusion is that it arises from surface roughness and/or heterogeneity. For surfaces that are not homogeneous, there exist domains that present barriers to the motion of the contact line. For example, hydrophobic do- mains will pin the motion of the water front as it advances, causing an increase in the observed contact angle; the same domains will hold back the contracting motion of the water front when the water recedes, thus leading to a decrease in the observed contact angle. In cases that surface roughness plays the role of generating hystere- sis, the actual microscopic variations of slope on the surface create barriers that pin the motion of the contact line and alter the macroscopic contact angles. Interpreting such contact angle data in terms of Young’s equation can be misleading because the equation fails to consider surface topography. Due to the complexity of contact angle phenomena, the experimentally observed contact angle might or might not be equal to Young’s contact angle θY [21, 22]. However, on ideal solid surfaces, there is no contact angle hysteresis, and the ex- perimentally observed contact angle is Young’s contact angle θY . On smooth but chemically heterogeneous solid surfaces, the experimentally observed contact angle might not be equal to θY . Nevertheless, the experimental advancing contact angle θa might be expected to be a good approximation of θY [21, 22], while the ex- perimental receding angle θr is usually found to have less reproducibility due to liquid sorption or solid swelling [23]. On rough solid surfaces, there is usually no correlation between θa and θY . All contact angles on rough surfaces are largely meaningless in terms of Young’s equation [21, 22]. The thermodynamic equilib- rium contact angles on rough and heterogeneous surfaces are called Wenzel [24] and Cassie-Baxter angles [25], respectively. They are not equivalent to Young’s contact angle. There are as yet no general guidelines regarding how smooth a solid surface must be for surface roughness not to have an obvious impact on the contact angle. It is therefore recommended that the solid surface should be prepared as smooth as possible, and as inert to the liquids of interest as possible. Several techniques for the preparation of smooth homogeneous solid surfaces have been developed; these include: heat pressing [26], solvent casting [27, 28], self-assembled monolay- ers [29, 30], dip coating [31, 32], vapor deposition [33, 34], and surface polishing [35, 36].